SegmentMerger doesn't set payload bit in new optimized code

Details

Description

In the new optimized code in SegmentMerger the payload bit is not set correctly
in the merged segment. This means that we loose all payloads during a merge!

The Payloads unit test doesn't catch this. Now that we have the new
DocumentsWriter we buffer much more docs by default then before. This means
that the test cases can't assume anymore that the DocsWriter flushes after 10
docs by default. TestPayloads however falsely assumed this, which means that no
merges happen anymore in TestPayloads. We should check whether there are
other testcases that rely on this.

The fixes for TestPayloads and SegmentMerger are very simple, I'll attach a patch
soon.